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JEE Main 2021
Mathematical Reasoning
Mathematical Reasoning
Easy

Question

If the Boolean expression (pq)(pq)(p \wedge q) \odot (p \otimes q) is a tautology, then \odot and \otimes are respectively given by :

Options

Solution

Key Concepts and Formulas

  • Tautology: A statement that is always true, regardless of the truth values of its components.
  • Implication: pqp \to q is equivalent to pq\sim p \vee q.
  • De Morgan's Laws:
    • (pq)pq\sim (p \wedge q) \equiv \sim p \vee \sim q
    • (pq)pq\sim (p \vee q) \equiv \sim p \wedge \sim q
  • Law of Excluded Middle: ppTp \vee \sim p \equiv T (True)

Step-by-Step Solution

Step 1: Assume \odot is \vee and \otimes is \to We want to check if the given expression is a tautology with these assignments: (pq)(pq)=(pq)(pq)(p \wedge q) \odot (p \otimes q) = (p \wedge q) \vee (p \to q)

Step 2: Rewrite the implication Using the implication equivalence pqpqp \to q \equiv \sim p \vee q, we can rewrite the expression: (pq)(pq)(p \wedge q) \vee (\sim p \vee q)

Step 3: Apply associativity and commutativity Rearrange the terms to group similar variables: (pq)(pq)p[q(pq)](\sim p \vee q) \vee (p \wedge q) \equiv \sim p \vee [q \vee (p \wedge q)]

Step 4: Simplify using the absorption law Note that q(pq)qq \vee (p \wedge q) \equiv q. Therefore, we have pq \sim p \vee q This is NOT a tautology, so this combination of \odot and \otimes won't work. Our assumption in Step 1 was incorrect.

Step 5: Assume \odot is \to and \otimes is \to We want to check if the given expression is a tautology with these assignments: (pq)(pq)=(pq)(pq)(p \wedge q) \odot (p \otimes q) = (p \wedge q) \to (p \to q)

Step 6: Rewrite the implications Using the implication equivalence pqpqp \to q \equiv \sim p \vee q, we can rewrite the expression: (pq)(pq)(p \wedge q) \to (\sim p \vee q)

Step 7: Rewrite the implication as a disjunction Using the implication equivalence again, we get: (pq)(pq)\sim (p \wedge q) \vee (\sim p \vee q)

Step 8: Apply De Morgan's Law Apply De Morgan's law to (pq)\sim (p \wedge q): (pq)(pq)(\sim p \vee \sim q) \vee (\sim p \vee q)

Step 9: Rearrange terms Use the associative and commutative properties of \vee: p(qq)\sim p \vee (\sim q \vee q)

Step 10: Apply the Law of Excluded Middle Since qq\sim q \vee q is always true (Law of Excluded Middle), we have: pT\sim p \vee T

Step 11: Simplify Since anything ORed with true is true, the expression simplifies to: TT

Step 12: Conclude that the expression is a tautology Since the expression is always true, it is a tautology. Therefore, \odot corresponds to \to and \otimes corresponds to \to.

Step 13: Assume \odot is \wedge and \otimes is \vee We want to check if the given expression is a tautology with these assignments: (pq)(pq)=(pq)(pq)(p \wedge q) \odot (p \otimes q) = (p \wedge q) \wedge (p \vee q)

Step 14: Expand and simplify (pq)(pq)(pqp)(pqq)(pq)(pq)pq(p \wedge q) \wedge (p \vee q) \equiv (p \wedge q \wedge p) \vee (p \wedge q \wedge q) \equiv (p \wedge q) \vee (p \wedge q) \equiv p \wedge q

This expression is only true when both pp and qq are true, so it is NOT a tautology.

Step 15: Assume \odot is \wedge and \otimes is \to We want to check if the given expression is a tautology with these assignments: (pq)(pq)=(pq)(pq)(p \wedge q) \odot (p \otimes q) = (p \wedge q) \wedge (p \to q)

Step 16: Rewrite the implication (pq)(pq)(p \wedge q) \wedge (\sim p \vee q)

Step 17: Distribute (pqp)(pqq)F(pq)pq(p \wedge q \wedge \sim p) \vee (p \wedge q \wedge q) \equiv F \vee (p \wedge q) \equiv p \wedge q This is only true when both pp and qq are true, so it is NOT a tautology.

Common Mistakes & Tips

  • Remember the truth tables and equivalences for logical operators.
  • De Morgan's Laws are crucial for simplifying expressions with negations.
  • When checking for tautologies, try to simplify the expression as much as possible.

Summary

We tested each of the options by substituting the logical operators into the given Boolean expression and simplifying. We found that when \odot is implication (\to) and \otimes is implication (\to), the expression simplifies to a tautology (always true). Therefore, the correct assignment is \odot as \to and \otimes as \to.

The final answer is \boxed{\to, \to}, which corresponds to option (B).

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